When seeking to increase the density of information recorded on an optical disk, one is generally limited by the performance of the information reading device. The basic principle is that only with very great difficulty can physical information registered in the disk be read if its dimension is lower than the limit of resolution of the optical system which will serve for reading this information. Typically, with reading by a red laser of wavelength 650 nm and a numerical aperture of 0.6, it is not normally possible to expect to correctly read information of dimension lower than 0.4 micrometers, strictly 0.3 micrometers.
However, so-called super-resolution procedures have been devised for reading information whose physical dimension is lower, indeed even much lower, than the wavelength. These procedures are based on the non-linear optical properties of certain materials. The expression non-linear properties is understood to mean the fact that certain optical properties of the material change as a function of the intensity of the light that they receive. The reading laser itself will locally modify the optical properties of the material in a reversible manner through its thermal, optical, thermo-optical and/or optoelectronic effects on smaller dimensions than the dimension of the laser reading spot; on account of the change of property, an item of optical information present in this very small volume becomes detectable although it would not have been detectable without this change.
The phenomenon utilized is based mainly on two properties of the reading laser that will be used:                on the one hand the laser is very strongly focused in such a way as to exhibit an extremely small cross section (of the order of the wavelength) but whose power distribution is Gaussian, very strong at its center, very attenuated at the periphery,        and on the other hand, a reading laser power is chosen such that the power density over a small part of the cross section, at the center of the beam, significantly modifies an optical property of the layer, whereas the power density outside of this small portion of cross section does not significantly modify this optical property; the optical property is modified in a direction tending to allow the reading of an item of information which would not be readable without this modification.        
For example, the optical property which changes is an increase in the optical transmission in the case where the reading of a bit consisting of a physical mark formed on the optical disk requires transmission of the laser beam up to this physical mark. The nonlinear layer is then interposed in the path of the beam to the physical mark. The center of the laser beam will be able to cross the layer up to the mark, on account of the fact that on crossing the layer the intensity of the incident light renders it more transparent, whereas the periphery of the beam will not cross since it does not sufficiently modify the optical indices of the layer to render it more transparent. Everything then occurs as if a beam had been used that was focused on a much narrower diameter than permitted by its wavelength.
Various theoretical proposals have been formulated for implementing these principles, but none has given rise to industrial development. U.S. Pat. No. 5,153,873 recalls the theory. U.S. Pat. No. 5,381,391 gives the example of a film having non-linear reflectivity properties. U.S. Pat. No. 5,569,517 proposes various materials having crystalline phase change.
Additionally, it is known that the reading of very dense digital information (marks very close together in the direction of travel of the marks under the laser beam or very short marks in this direction) is difficult to achieve without error by simple detection of an electrical voltage threshold being overshot by the signal arising from the reading.
Specifically, even assuming that it is possible to record marks of properly square shape defining 0 or 1 binary information (for example 0 in the absence of a hole in a physical layer, 1 in the presence of a hole), the reading signal which results from these marks traveling past is not a square signal but a deformed signal on account of the limited bandwidth of the reading system. And in any event, it is not known how to record perfectly square marks on account of the physical procedures employed for recording these marks.
The combination of this imperfection of the physical marks and of the imperfection of the reading systems gives rise to a voltage signal of very degraded form for representing an item of information which ought to be very square (purely binary information). And this voltage signal of very degraded form is all the more deformed and difficult to interpret the closer together the marks; for example, instead of having very marked voltage spikes, well localized in time at each binary transition, and easy to detect by a voltage threshold detector, it is appreciated that the reading signal comprises hollows or bumps that are less marked at the locations of these binary transitions; the amplitude of these hollows or these bumps and their temporal localization are moreover very dependant on the succession of marks which precedes the mark that one wishes to detect.
In the prior art, more sophisticated procedures for the electronic processing of the reading signal have been proposed; these procedures allow better decoding, for a given information density, of the binary information on the basis of a very deformed reading signal, or alternatively they make it possible to record and read out information at a higher density than that permitted by the procedures with simple threshold detection. These more sophisticated procedures are called PRML procedures, the abbreviation standing for “Partial Response Maximum Likelihood”. They rely on:                the theoretical estimation of the forms of response that may be taken by the signal at the output of the processing channel when this channel receives various models of successions of binary information (Partial Response PR),        and on the comparison between the output signal actually detected and the various theoretical forms of response so as to determine which succession of binary information is the one that was most probably emitted at the input to the channel to give this signal on output (Maximum Likelihood ML).        
When one speaks of a channel for processing binary information, this implies all of the electronic and physical or chemical processing going from the writing of the binary information to the disk to the reading of this information in the form of an analog electrical signal. Indeed, it is upstream of the writing that a binary item of information to be recorded is available, but the writing itself degrades the binary nature of the information (the physical marks recorded have shapes that are not rectangular) and the reading also degrades, and generally even more, this nature.
In the reading systems envisaged for reading optical disks, whether it be with a red laser or with a blue laser (the blue laser allowing reading of information of higher resolution), it has been proposed to use PRML procedures in which the response waveform of an isolated information bit (in practice an isolated binary transition) is regarded as a Gaussian shape centered on a characteristic instant defining the temporal position of the bit. This Gaussian shape is thereafter modeled by a succession of P samples of nonzero values taken from N possible values. The numbers P and N are small (a few units) so that the PRML calculations are reasonable in terms of quantity; an approximation by a larger number of values N is more exact but requires more calculations; an approximation with a larger number of samples P would be better but necessitates a larger sampling frequency and hence faster calculations. Given that the numbers N and P are small, the model obtained is very Spartan and the term “caricature” will sometimes be used to denote this model of a binary response or the model of a response to a succession of bits, and the verb “to caricature” will be used to denote the calculation of a theoretical model of an information bit or of a succession of information bits.
FIG. 1 represents a typical example of a Gaussian theoretical shape of response for an isolated binary transition, as well as the caricatural model using P=4 nonzero samples that can take only N=2 standardized values 1 and 2. These values P=4 and N=2 are conventionally used, the Gaussian being caricatured by the succession of four successive nonzero digital values, namely 1,2,2,1. The sampling frequency is F, and the sampling period is T=1/Fe (one sample at each period T). For appropriate reading of the binary information using a PRML procedure with this type of form of theoretical Gaussian analog response for an isolated binary transition and this type of digital modeling of the response, provision may be made for the sampling period to be equal to a quarter of the mid-height width of the Gaussian curve. This is what is represented in FIG. 1. And it is then appreciated that it is possible to recover a binary information item recorded on the disk on condition that the successive binary transitions are spaced apart by at least twice the sampling period (2T). Below this value, the transitions would be too close together to be able to be read with sufficient safety.
Another example of Gaussian waveform caricature is represented in FIG. 2 with P=5 and N=2. The model or caricature is now 1,2,2,2,1. It gives slightly better results than the 1,2,2,1 model and is proposed, like the previous one, in the BD (Bluray Disk) and HD_DVD ROM standards.
FIG. 3 (3A to 3F) recalls the principle of a PRML procedure.
Represented in line 3A is a binary information sequence to be written, coded by a variable number of bits which preserve one and the same value between two binary transitions (RLL code). The duration of a bit when reading back the recorded information is assumed to be equal to the sampling period T used in the PRML procedure, but the binary sequence is conventionally such that there are always at least two identical consecutive bits.
Represented in line 3B is the succession of physical marks recorded on the basis of this sequence: marks of length corresponding to the number of bits between two transitions, followed by an interval between marks, the interval having a length corresponding to the number of bits before the next transition. The lengths of marks, like the distances between marks, then represent, under a code other than the starting RLL code, the binary information stored. The length of a mark may be expressed as a duration, and more precisely as an integer number of periods T, the marks traveling past at constant speed under the reading laser beam, the value T representing the duration of a bit.
Represented in line 3C is the coded digital information corresponding to the physical marks: a mark present is a 1, an absence of mark (or intermark or mark of inverse polarity) is a zero.
Represented in line 3D is the conventional response model for an isolated one 1 bit; the example chosen is that of the 1,2,2,1 model of FIG. 1; the response for a zero bit (absence of mark) is assumed to be zero.
Represented in the group of lines that is denoted 3E is the succession of successive digital models 1,2,2,1 that will engender the presence of each of the bits of each of the successive marks while the length of the model (4T) is greater than the length of a bit (T): the models overlap and the model resulting from a succession of 1 bits is the addition of the digital values shifted in time resulting from these multiple overlaps. The result of this digital addition is registered on the last line of group 3E.
Represented in line 3F is the temporal succession of the digital values resulting from this overlap and from this addition. The digital values extend over a scale ranging from 0 to 6. The number 6 being the ceiling which results from the overlap for the values P=4 and N=2. The scale would go from 0 to 8 for P=5 and N=2.
Represented in line 3G is an actual reading signal which corresponds to the reading of the marks which were recorded on the basis of the binary sequence of line 3A, as well as the digital values resulting from a sampling of this signal at the frequency Fe=1/T where T is the theoretical duration of a bit, having regard to the length of a physical mark corresponding to a bit and the speed of rotation of the disk under the reading laser beam. The reading signal is standardized with a scale (0 to 6) similar to that of the theoretical model of FIG. 3F so that comparison is possible.
The PRML technique consists in gathering a succession of K samples of the analog signal resulting from an actual reading; in calculating all kinds of predetermined theoretical successions of the type of that of line 3A (hence for all kinds of possible binary successions which could have been recorded although it is not known which one was really recorded); in measuring the resemblance between the succession received and each of the successions calculated, and in deducing therefrom which binary sequence was probably the starting one given the better resemblance found.
The resemblance is calculated preferably by the so-called least squares procedure in which:                for a determined succession of samples, the sum is calculated of the squares of the differences between each sample of the succession received and the corresponding sample of the succession calculated,        this is repeated for all the possible successions calculated,        the various sums of squares calculated are compared,        and the binary succession which gives the smallest sum of squares is selected from among all the possible binary successions; it is considered that this binary succession is indeed the sequence which was recorded in the disk, because the theoretical response calculated for this succession is that which most resembles the reading signal on the basis of the least squares criterion.        